in traditional “plain-old-telephone-systems” (POTS systems), it was relatively easy to implement 911 calling service because every incoming phone line (with the exception of long-ago-abandoned “party lines”) corresponded to a physically connected analog phone. This was particularly true in residential systems, such that upon receiving a “911” call at the local telephone office, the caller location would be immediately known and could be conveyed to emergency responders. Specifically, with “enhanced 911” (e911) services, the telephone number of the caller is provided, and an Automatic Location identification(ALI) database is used to look up the caller's street address and name information. This all works well in the fixed-line context, but as will be seen below, there have been shortcomings in the context of enterprise- or “campus”-type phone systems.
Enterprise or campus-type phone systems (businesses, hospitals, schools, etc.) also have emergency call routing needs, but such phone systems are typically “multiple line telephone systems” in which multiple analog or digital phones are connected with a “private branch exchange” or “PBX” phone switch. These systems have existed for decades, but there have not been effective ways in such systems to relay the specific campus location of a caller from such a system to an emergency responder. Until fairly recently, the only address sent to an emergency responder would have been the address associated with the “headquarters” or security desk associated with the PBX, although it is also possible for an enterprise to build its own ALI database information to the extent there is a dedicated phone number (or “Direct Inward Dial” or “DID” number) associated with each caller. But generally, the emergency responders would have to coordinate with the campus or enterprise security department, who also would lack specific location information on the emergency caller and would have had to use off-line methods (paging, sending out a patrol, etc.) to try to locate the emergency caller. Also, these human entries into the ALI or other database are subject to human error, which can further frustrate efforts to locate emergency callers.
Now with the advent of Voice-over-IP phones (“VoIP”) that can be plugged into any network jack on an enterprise campus, any ALI database “maps” of phone locations that are maintained by the enterprise are subject to being inaccurate due to VoIP phones (either wired or wireless) being moved around on the campus. Attempts to address this problem have focused on placing location information into the VoIP phones themselves so that information can be transmitted with an emergency call. An example of this approach is a published patent application assigned to Lucent, US 2006/0293024 A1, to Benco et al. This application describes embodiments in which GPS tracking equipment is placed in the VoIP phone. But GPS does not generally work inside buildings, particular in the interior areas of the buildings, and it would not provide “floor” information within a multi-story building. And even GPS tracking information, although relatively accurate, still has an uncertainty window around it of about 15-30 feet. This variability is still enough to put emergency responders in the wrong room, wrong floor, or even the wrong building using GPS location information transmitted from VoIP phones as a part of an emergency call.
Another prior-art approach is described in a published patent application, US 2007/0242660 A1 to Xu. This application describes a system in which a VoIP phone (VoIP endpoint) determines to which network device and port number it is connected. Thus, presuming the location of the endpoint device is stored in an ALI database, a general location of the VoIP phone could be determined by knowing where the network device is located. Shortcomings associated with this approach, however, include the fact that network devices are frequently upgraded or replaced, and they are generally kept in a rack in a communications “closet,” and there might be one or fewer such closets on each floor of a corporate building, and so keeping a database of the network devices and their port locations is prone to human error and lack of diligence in keeping that database up-to-date, and the location of a given network device will not provide sufficiently specific information for an emergency responder.